A transmission is incorporated in a vehicular drive train between the engine and the drive wheels to provide a plurality of gear ratios for moving forwardly and generally one gear ratio for moving rearwardly. The transmission is a necessary part of a vehicular drive train, because internal combustion engines can deliver only limited torque at low revolutions per minute (RPM). The transmission allows the speed of the engine to be maintained within its optimum operating range for the delivery of maximum torque or power, as the vehicle accelerates from a stationary or "stopped" position to the desired speed. The speed reduction between the RPM of the engine and the resulting rotation of the drive wheels provided by a transmission effects a controlled application of the torque by which the drive wheels are rotated. Accordingly, when that gear ratio commonly designated as "low" or "first" gear is selected, the transmission imparts less speed to the drive wheels, but imparts more torque from the engine to rotate the drive wheels in order to overcome the static inertia of the vehicle and effect forward movement. As the speed of the vehicle increases, the transmission may be selectively shifted through the plurality of gear ratios it provides in order to impart progressively greater rotational speed to the drive wheels with concomitantly lesser torque.
Many manual transmissions provide three forward gear ratios. These transmissions are generally identified as "three-speed" transmissions, but "four-speed" and "five-speed" transmissions are also quite common, because they permit an engine to operate within a smaller optimal speed range, while effecting progressively increasing forward speed to the vehicle. Irrespective of the exact number of gear ratios provided, manual transmissions offer a plurality of forward speed gear ratios from which the driver may select--though normally the selection is sequential--to transmit torque from the engine to the drive wheels. Generally, only a single reverse gear ratio is provided. When using a manual transmission the selection of a specific gear ratio is made by the driver.
Transmissions which employ an even number of forward gear ratios--such as a four-speed transmission--generally effect the shift between the first and the second gear ratios as well as between the third and the fourth gear ratios by movement of the shift selecting lever in at least two separate planes. In such transmissions, the aforesaid two planar movements are laterally displaced, one with respect to the other, and the shift selecting lever is capable of being laterally displaced between the lateral locations of the planar movements which effect the shift into the desired forward gear ratio only when the shift lever is in "neutral".
To select the reverse gear, the shift selecting lever is moved into an even further lateral displacement, also when the shift selecting lever is in neutral, and then moved within a third plane. This third plane is parallel to those planes within which the shift selecting lever is moved to effect selection of the forward drive ranges. In such an arrangement, it is impossible to move the shift selecting lever within a common plane between any of the forward gear ratios and the reverse gear ratio, or vice versa.
On the other hand, in transmissions which employ an odd number of forward gear ratios, the lateral displacement of the shift selecting lever required for shifting into reverse is often coincident with that lateral displacement of the shift selecting lever required for shifting into one of the forward gear ratios. With the planar movement required to shift into reverse being thus aligned with the planar movement to shift into one of the forward drive ratios, it would be possible to shift from a forward gear ratio directly into reverse, unless some mechanism, or structure, has been included in the transmission to preclude inadvertent movement therebetween.
The prior art knows many structural arrangements which preclude inadvertent shifting between one of the forward gear ratios and the reverse gear ratio, and, for the most part, they have been quite successful in effecting the desired result. Examples are the usage of a button or lever, which will disengage a reverse inhibitor, allowing the shift mechanism to engage reverse gear. This mechanism, however, not only adds to the cost of the transmission, inasmuch as it requires a great number of parts which require additional machining to insure the desired accuracy and proper operation of the transmission, but is also of at least modest inconvenience to the driver, in view of the additional manipulations required to effect a shift into reverse.
Another structural arrangement employed to preclude inadvertent shifting between the forward and reverse ranges, even with an odd number of forward shift ranges, requires that the shift lever be laterally displaced into a distinct planar path which is reserved for shifting into the reverse range. Heretofore, this arrangement has not gained wide acceptance because it was relatively complex and required what is deemed to be too many parts, and, when implemented, was considered to be moderately inconvenient to the driver.